In the manufacture and assembly of electric and electronic units, wiring boards have long been used to interconnect electronic components. These “interconnection” boards or cards have an insulating substrate with a plurality of electronic components, which may be integrated circuit packages, or other types of electronic, electro-optical or optical components, mounted thereto and a pattern of conductive path segments connecting components to one another.
Because optical filaments or optical fibers can transmit much more information (and with significantly less signal degradation) than electrical conductors, their use is growing. For example, optical backplanes increasingly are used in electronics systems where greater circuit density is desired, but is difficult to provide with known electrically wired backplanes. An optical backplane is formed by a plurality of optical filaments mounted or routed on a substrate in a given pattern or circuit geometry. Optical backplanes interconnect optical circuit components, which transmit signals optically, as well as electrical circuit components, wiring boards, modules and/or integrated circuits. When an optical backplane interconnects electrical components, the electrical energy of each component is translated to optical energy which is transmitted by optical filaments on the optical backplane to another electrical component where it is translated back to electrical energy for transmission to the other electrical component.
Optical backplanes are fabricated according to various methods, ranging from laying the optical filaments on the substrate by hand to routing the optical filaments in a given pattern onto the substrate by mechanized apparatus. However, commercial interconnection card manufacture with optical filaments is difficult and these known methods often lead to unacceptable results. Optical filaments are extremely small and difficult to handle. These tiny filaments also are fragile, and often cannot withstand stresses attendant to abrupt turns or the like during the scribing process. Optical filament breakage at crossovers (i.e., where one wire overlaps itself or another wire) is particularly problematic, because stresses at these points can increase dramatically. This can be even more troublesome when ultrasonic energy is used to adhere the filaments to the substrate, which can lead to further filament breakage. As breakage rates increase, the commercial viability of optical filament circuit board manufacture decreases.
For example, U.S. Pat. Nos. 3,674,602 and 3,674,914, both dated Jul. 4, 1972, describe interconnection card manufacturing methods. According to these methods, a substrate is mounted on a vacuum table. Then a wire is scribed onto a substrate surface in a pre-selected planar circuit pattern using a wire dispensing and bonding head. This method is known as “wire-scribing.” An adhesive film laminated to the substrate surface secures the wire to the substrate. Energy emitted by the bonding head actuates the adhesive film as the wire contacts therewith, so that the wire bonds to the substrate.
However, during wire scribing, the stresses can be so severe (reaching up to up to 12 MPa at crossovers) that the wire conductor ruptures. In fact, U.S. Pat. No. 5,483,603, issued to Luke et al. and dated Jan. 9, 1996, describes a system and method of automatic optical inspection of wire-scribed circuit boards to detect breakage. Similarly, the stress on optical filaments during scribing increases the possibility of breakage, and “microbending,” a condition that causes signal attenuation along the optical filament.
U.S. Pat. Nos. 6,088,230 and 6,233,818, dated Jul. 11, 2000 and May 22, 2001, respectively, describe other methods of bonding wire conductor to a substrate or chip-mounting board. The '230 patent discloses that a chip is mounted onto a substrate by an adhesive layer applied to the substrate. Then, a free wire end is connected to a chip contact surface by soldering. Then the coil wire is dispensed onto the substrate surface. At least at some points the wire is fused to the substrate. Finally, a second free wire end is connected to another chip contact surface by soldering. The '818 patent discloses a similar method, which allegedly is applicable to glass fibers, in additional to metallic conductors. However, the '230 patent and the '818 patent fail to describe a single embodiment using optical filaments or provide process parameters for successfully scribing optical filaments according to these methods.
U.S. Pat. No. 5,259,051 to Burack et al., dated Nov. 2, 1993, discloses a method of making optical filament interconnections by routing optical filaments on a substrate. Because the scribing head is capable of X, Y and Z-axis motion, crossovers can be formed. A spring pushes the scribing head down onto the substrate and permits the head to be pushed up at crossovers when obstacles are encountered. Z-axis movement at the optical filaments is not controlled, but instead relies on resistance caused by the obstacles encountered during scribing.
Also, the optical filaments are not embedded into the substrate in the method of the '051 patent, but rather are laid onto the substrate's surface. Accordingly the optical filaments are covered after routing by a plastic sheet that encapsulates the fibers. The sheet is laminated onto the substrate by application of pressure and heat. This pressure, which is necessary to encapsulate the optical filaments, also may break the optical filaments. Breakage is especially a problem at crossovers , which are increasingly common and necessary in optical filament circuit cards.
U.S. Pat. No. 5,292,390 to Burack et al., dated Mar. 8, 1994, discloses a similar method of producing optical circuits. Optical filaments first are bonded to a substrate's upper surface and then are covered with a thermoplastic sheet material. The resulting structure then is compressed at an elevated temperature and relatively high pressure to bond or tack the thermoplastic material to the plastic substrate. After cooling, the structure again is heated and subjected to elevated pressure to cause the thermoplastic material to encase the optical filaments. Again, the fibers in this method still are susceptible to damage, particularly when many crossovers are included in the optical filament circuit pattern, because heat and pressure are applied to the thermoplastic sheet material.
Accordingly, there has been a long-felt need in the industry for methods of manufacturing optical filament circuit boards that are amenable to mass production. Various machines are available for automatically routing and bonding electrical wire to a substrate, but, in general, these machines cannot be adopted for optical filament use because optical filament is relatively fragile and is relatively unable to withstand heat and pressure, abrupt turns, etc.